1. Field of the Invention
The instant invention is directed to the plastic laminate of two or three plys useful in manufacturing plastic fabricated articles, especially containers, and more particularly, beverage containers. More particularly, the present invention relates to a two- or three-ply laminate, each such laminate including a layer of polyethylene terephthalate, characterized by the inclusion of a gas barrier layer which is well suited for holding carbonated beverages.
2. Background of the Prior Art
The phenomenal growth in the use of plastic containers to hold foods, household aids and the like evidences its unique suitability to these applications. Among these commercial successes, the use of plastic bottles to contain carbonated beverages has been particularly noteworthy. The reason, for this commercial success has been ascribed to the principal resin utilized in this application, polyethylene terephthalate (PET). PET, when biaxially oriented, produces a bottle whose advantages include low cost, light weight, durability, rigidity and clarity.
PET, and bottles made therefrom, however, is characterized by one major detrimental property which is important insofar as carbonated beverages are concerned. That detrimental property is gas permeability. Carbonated beverages, of course, depend on the maintenance of a requisite concentration of carbon dioxide gas pressure to maintain the carbonated quality of the beverage. However, a bottle formed solely of PET permits loss of carbonation because this resin provides only a limited barrier to escaping carbon dioxide gas. Of less importance but of some significance is the fact that this gas permeability also permits introduction of oxygen. Oxygen degrades taste. However, long shelf life, that is the retention of carbonation over long periods of time, represents a major problem associated the use of PET bottles employed to contain carbonated beverages.
Indeed, gas permeability has prevented the widespread use of low volume PET carbonated beverage bottles, albeit they are widely used as high volume containers. That is, the use of PET bottles is usually limited to bottles which contain at least 1 liter of carbonated beverage. This, of course, is due to the relationship between beverage volume and the unit surface area of the bottle. The greater the beverage volume, the greater is the ratio of beverage volume to surface area of the bottle. Those skilled in the art are aware that the greater this ratio the lesser is the degree of gas loss, in this case, carbonation. However, the limited nature of the gas barrier property provided by PET has prevented the use of these bottles in smaller volume containers.
An obvious solution of this problem is a laminated container which includes a layer of a resin having little or substantially no permeability to gases, that is, a gas barrier layer. Indeed, a plurality of references disclose PET bottles, used to contain carbonated beverages, which include a gas barrier layer. Some of the more important of these references include U.S. Pat. No. 4,284,671 to Cancio et al.; U.S. Pat. No. 4,482,586 to Smith et al.; U.S. Pat. No. 4,564,541 to Taira et al.; U.S. Pat. No. 4,601,926 to Jabrin et al.; and U.S. Pat. No. 4,618,386 to Yatsu et al.
A major drawback to the advance in the art provided by these patents, and many other references which can be cited, is the complexity of the laminate structure required to eliminate gas permeability of PET-containing bottles. This complexity is manifested by preferred embodiments mentioned therein which include at least five plys.
The requirement in many of these references that the laminate include at least five plies is caused by the non-compatibility between the gas barrier layer and the polyethylene terephthalate layer. Thus, the laminates within the contemplation of the these patents requires that the gas barrier layer be bonded, on both sides, with an adhesive which, in turn, is bonded to an inner layer of PET and to an outer layer which may be PET or the like.
A preferred gas barrier layer used in many of these references is a polar resin, i.e. polyvinylalcohol (PVOH) or ethylene-vinyl alcohol copolymer (EVOH). Thus, a preferred laminate utilized in forming a beverage bottle in accordance with many of these patents includes a PET outer layer, an adhesive layer, a layer of a gas barrier polar resin, another adhesive layer and finally a PET inner layer.
Some of these references attempt to overcome this obvious disadvantage by blending the gas barrier resin with a resin compatible with PET so that the number of plies can be reduced. This expedient is certainly advantageous. However, this is ineffective in most cases since the preferred gas barrier resins are polar. Specifically, as stated above, most references directed to PET bottles which utilize a gas barrier resin include poly(vinyl alcohol) (PVOH), ethylene-vinyl alcohol copolymers (EVOH) or the like as the gas barrier resin layer.
Those skilled in the art are aware that for economic and ecological reasons PET bottles are recycled and reused in freshly made bottles. To that end the used bottle is ground into chips, powders and the like and melted. Commonly employed vinyl alcohol-containing polymers are incompatible with molten PET. As such, the use of bottles which contain polar gas barrier resins even if employed in direct contact with PET cannot be recycled to produce new PET carbonated beverage bottles.
The above problems, associated with the development of a PET-containing laminate plastic carbonated beverage bottle, has suggested the substitution of other gas barrier polymers. One such class of polymers, known to possess moderate to high gas barrier properties, are the so-called "phenoxy-type" thermoplastics. A specific class of such polymers, including poly(hydroxy ethers), poly(hydroxy ester ethers) and poly(hydroxy amino ethers), is described in White et al., Polymer Preprints, 34(1), 904-905 (1993). These phenoxy-type thermoplastics are recited therein to have good oxygen barrier properties combined with excellent mechanical properties. The paper indicates that such polymers can be fabricated into films and containers. However, no suggestion is given in this reference of utilizing such polymers with thermoplastic polyesters in general or PET bottles in particular.
The above discussion suggests the need in the art for an improved laminate structure which can be utilized to make bottles holding small volumes of carbonated beverage which are easy to fabricate, eliminate carbonation loss, are characterized by the excellent properties associated with PET bottles presently used in the formation of large volume carbonated beverage bottles and employ compatible resins which are capable of being recycled.